The Wind Powered Sail-less Boat

Discussion in 'Boat Design' started by DuncanRox, Oct 20, 2008.

  1. ThinAirDesigns
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    ThinAirDesigns Senior Member

    Well, it never acts as a turbine, but it's definitely *part* of a "power recovery system". In this case, the prop does some work on the air (speeds it up relative to the cart) while the system has some work done *on it* by the air (which slows the air down relative to the ground).

    In the end it is the kinetic energy removed from the air relative to the ground that is used to motivate the cart.

    And again just to be clear, while in operation the cart leaves in its wake wind that is moving slower relative to the ground than it was before encountering the cart prop.

    JB
     
  2. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    Yes it does. Thanks a lot Rick for your patience (and the others also). I've been perfectly 'thick' on this matter and I apologize for it.

    I will eat my hat!

    Best regards.
     

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  3. robherc
    Joined: Dec 2008
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    robherc Designer/Hobbyist

    Looks to me like you're starting to understand QUITE well actually and, as it took me a long thought to get past where you are now, I THINK I can help you...

    If you figure that the amount of propulsion of any propeller results from its ability to "push" air towards the opposite direction as the propeller is attempting to travel, it makes this easier.
    1.) At moment 0 (before ANY motion starts in the cart) the propeller is generating drag, and what little lift it creates is in the wrong direction, which is why the gearing to the wheels is important here.
    2.) At the moment that forward (downwind) motion begins, the wheels overpower the torque on the prop, and start spinning it in the proper direction (forcing air towards the rear). At this point, the propeller is not forcing any air to actually move behind the cart, just slowing the air down as it passes the cart. It is still acting as a propeller as it is still pushing against the wind, just that the wind is still pushing harder.
    3.) (this, to me, seems to be where the hardest point in the whole system would be) At some moment before the cart reaches windspeed, the propeller will be pushing back on the air with a force exactly equal to the force the air is pushing forward. Here there will be (it seems) some stagnant, pressurized, air gathering behind the prop...i.e. relative air-cart velocity here=zero.
    4.) After the point of stagnation is overcome, the air immediately behind the prop. blades will actually be moving BACKWARDS relative to the cart. This is the point where it should become easier for you to see the propeller as becoming functionally a propeller, instead of simply a spinnaker. From this point on, the actual acceleration speed the prop. is imposing on the "wash" air is:
    Va+Vpa=Vc
    where Va is the FORWARD air velocity relative to the cart (wind speed), Vpa is the BACKWARD air velocity relative to the cart (prop wash), and Vc is the acceleration on the wash imposed by the prop.
    5.) At some point prior to the cart achieving DDW velocity equal to wind speed, the equation inputs can be changed to speeds relative to the ground (instead of the cart), as the wind speed relative to the cart is slowing towards zero, and the wash speed relative to the ground has reached zero...and is now building in the REVERSE direction of the wind. (this is the point where a stationary flag that {theoretically} was placed behind the cart in motion, would be blown backwards, away from the cart, by the prop. wash.
    6.) When the cart's downwind speed exactly matches the windspeed, there will be zero wind relative to the cart, so the prop would be functioning (for propulsion purposes) identically to the prop. of an airplane the instant the plane reaches wind velocity while making a downwind takeoff! (though any competent pilot would avoid this situation as it yields ground speed>air speed and makes it take more ground distance to build significant airspeed & lift on the wings)
    7.) At all points after the cart has reached DDWFTTW velocities, the prop will perform exactly as a prop on an airplane flying with the wind (DDWFTTW) and gaining a "speed boost" from the "tailwind"
     
  4. spork

    spork Previous Member

    ...relative to the ground. The air is actually sped up relative to the cart. That's part of what makes this such a mind bender.
     
  5. Guest625101138

    Guest625101138 Previous Member

    No need to eat your hat. Just the acknowledgment that it is possible is sufficient. It may help others to take a bit more time to think about it before they chime in with ill thought statements and closed minds.

    I am looking forward to a boost in reputation based on the patience I have shown and even temper over some 30 odd pages - well almost even temper.

    Rick W
     
  6. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Now that we have finally convinced Guillermo, I would like to go back to autorotating. Have you had time to think about it? I'm still sure, that a parachute will have (clearly) more drag than an autorotating rotor of same disc area in a case of purely axial flow.

    Joakim
     
  7. Guest625101138

    Guest625101138 Previous Member

    Joakim
    As you have a good grasp on the physics and the actual design of props and turbines I would appreciate it if you critique my energy balance in post #495. The energy taken from the air stream was more than I needed and I could not work out where it all goes so I introduced the Epl factor. Somehow this does not make sense because it is greater than the power input to the prop.

    I am confident about the operating points for the turbine and prop as they are based on proven models but I am not sure about my calculation of the energy that the air gives up. It is not often you get more energy than you need.

    Rick W
     
  8. Jimbo1490
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    Jimbo1490 Senior Member

    Looking at the pics and videos of these carts, it's apparent to me that the main source of the simple drag which gets the cart rolling and powers the propeller is the propeller itself. The rest of the cart is only a few upright posts; the significant drag comes from those big flat prop blades. This continues to be true after the cart starts to roll, also. So the propeller is 'climbing' into the air mass in front of it even while it presents a significant drag to the air approaching from behind. At first this seems irreconcilable, but it's really all about which point of view you choose to adopt to analyze the forces.

    I bet if you minimize drag on the rest of the cart, it will still work as advertised because it's the drag on the back of the prop that counts most.

    Jimbo
     
  9. spork

    spork Previous Member

    My concern is less with the question of parachute vs. autorotating blades, and more with the question of whether an autogyro is inherently different than a helicopter in autorotation. I'm confident the two are the same. I haven't given much thought to the first question, but it's always been my understanding (sort of rule of thumb) that an autorotating blade system provides approximately the same drag as a parachute of the same diameter.

    I understand your thinking, and it makes sense to me. Without giving it a lot more thought I'm sure I couldn't add much to the discussion.
     
  10. spork

    spork Previous Member


    Yes, I'm confident that if we were able to eliminate the aero drag from the rest of the cart it would have little effect. It certainly wouldn't make a significant change in its ability to self-start in a tailwind.
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    Well...errr...I'm afraid not completely. :)

    I still have some strong disagreements on the DDWFTTW cart. What I have just understood is how all of you consider the rotor issue.

    But it's late today. I will chime in with more tomorrow.

    Best to all of you. Have a nice night, day or whatever.
     
  12. spork

    spork Previous Member

    I think you mean that you have disagreements regarding the prop-cart architecture. You've told us more than once that you agree a DDWFTTW can be made by hooking two ice-boats together with a telescoping pole.
     
  13. Jimbo1490
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    Jimbo1490 Senior Member

    I watch from 50 to 100 autorotations every day (when I bother to look up anymore) and they are the same for (true) helicopters as for autogyros. If you trace the development of helicopters, you'll find that just prior to the ascendency of the 'true' helicopters, more advanced autogyros had begun to 'spin up' their rotors while stationary to reduce or even completely eliminate the ground takeoff roll. With only rotor inertia, they could jump into the air and pick up forward speed (thus replenishing rotor energy) before they began falling again. The catch was that the rotor was brought up to speed with a belt drive or gearbox, but the power was disengaged from the rotor just prior to pulling the collective pitch, so that no torque moment was generated by the rotor disc to be transmitted to the axle/drive shaft, thus no need for a tail rotor. This was very convenient since the anti-torque rotor had not been worked out yet:D Sikorski did that later.

    I brought this little piece of history up to show that autorotation in a true helo is exactly the same as for an autogyro; in fact all the major issues with the helicopter rotor system (lead-lag, flapping vs rigid, teetering , 'coning' and etc) were all worked out in autogyros first.

    Jimbo
     
  14. ThinAirDesigns
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    ThinAirDesigns Senior Member

    Seems Guillermo is holding out on us over here on Boatdesign.net

    http://www.advancedphysics.org/forum/showthread.php?t=10550

    Either that he's flying some sort of double flag ... agreeing that it works over there and denying it here. His last post here showing disagreement is stamped very near the time he post over there agreeing that the cart works.

    A very difficult gentleman to get a read on as he won't engage to any degree.

    JB
     

  15. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    Only from time to time. You have been slow.... :)

    Cheers.
     
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